Add collections.Reversible. Patch by Ivan Levkivskyi. Fixes issue #25987.

2 files changed, 29 insertions, 4 deletions

diff --git a/Lib/test/test_collections.py b/Lib/test/test_collections.py
index 4c32e09..4202462 100644
--- a/Lib/test/test_collections.py
+++ b/Lib/test/test_collections.py
@@ -20,7 +20,7 @@ from collections import UserDict, UserString, UserList
 from collections import ChainMap
 from collections import deque
 from collections.abc import Awaitable, Coroutine, AsyncIterator, AsyncIterable
-from collections.abc import Hashable, Iterable, Iterator, Generator
+from collections.abc import Hashable, Iterable, Iterator, Generator, Reversible
 from collections.abc import Sized, Container, Callable
 from collections.abc import Set, MutableSet
 from collections.abc import Mapping, MutableMapping, KeysView, ItemsView
@@ -689,6 +689,31 @@ class TestOneTrickPonyABCs(ABCTestCase):
         self.validate_abstract_methods(Iterable, '__iter__')
         self.validate_isinstance(Iterable, '__iter__')
 
+    def test_Reversible(self):
+        # Check some non-reversibles
+        non_samples = [None, 42, 3.14, 1j, dict(), set(), frozenset()]
+        for x in non_samples:
+            self.assertNotIsInstance(x, Reversible)
+            self.assertFalse(issubclass(type(x), Reversible), repr(type(x)))
+        # Check some reversibles
+        samples = [tuple(), list()]
+        for x in samples:
+            self.assertIsInstance(x, Reversible)
+            self.assertTrue(issubclass(type(x), Reversible), repr(type(x)))
+        # Check also Mapping, MutableMapping, and Sequence
+        self.assertTrue(issubclass(Sequence, Reversible), repr(Sequence))
+        self.assertFalse(issubclass(Mapping, Reversible), repr(Mapping))
+        self.assertFalse(issubclass(MutableMapping, Reversible), repr(MutableMapping))
+        # Check direct subclassing
+        class R(Reversible):
+            def __iter__(self):
+                return iter(list())
+            def __reversed__(self):
+                return iter(list())
+        self.assertEqual(list(reversed(R())), [])
+        self.assertFalse(issubclass(float, R))
+        self.validate_abstract_methods(Reversible, '__reversed__', '__iter__')
+
     def test_Iterator(self):
         non_samples = [None, 42, 3.14, 1j, b"", "", (), [], {}, set()]
         for x in non_samples:
@@ -842,14 +867,14 @@ class TestOneTrickPonyABCs(ABCTestCase):
         self.validate_isinstance(Callable, '__call__')
 
     def test_direct_subclassing(self):
-        for B in Hashable, Iterable, Iterator, Sized, Container, Callable:
+        for B in Hashable, Iterable, Iterator, Reversible, Sized, Container, Callable:
             class C(B):
                 pass
             self.assertTrue(issubclass(C, B))
             self.assertFalse(issubclass(int, C))
 
     def test_registration(self):
-        for B in Hashable, Iterable, Iterator, Sized, Container, Callable:
+        for B in Hashable, Iterable, Iterator, Reversible, Sized, Container, Callable:
             class C:
                 __hash__ = None  # Make sure it isn't hashable by default
             self.assertFalse(issubclass(C, B), B.__name__)
diff --git a/Lib/test/test_functools.py b/Lib/test/test_functools.py
index 31930fc..a22d2e6 100644
--- a/Lib/test/test_functools.py
+++ b/Lib/test/test_functools.py
@@ -1516,7 +1516,7 @@ class TestSingleDispatch(unittest.TestCase):
             m = mro(D, bases)
             self.assertEqual(m, [D, c.MutableSequence, c.Sequence,
                                  c.defaultdict, dict, c.MutableMapping,
-                                 c.Mapping, c.Sized, c.Iterable, c.Container,
+                                 c.Mapping, c.Sized, c.Reversible, c.Iterable, c.Container,
                                  object])
 
         # Container and Callable are registered on different base classes and